(1) Field of the Invention
The present invention relates to a tubular blanket to be used for a blanket barrel (cylinder) of a printing machine, and to a printing machine using the tubular blanket.
(2) Description of the Related Art
A printing unit of an offset printing machine has basically been composed of a blanket and a printing cylinder, and recently, tubular blankets have come into widespread use.
For explanation of a tubular blanket, a printing unit of a blanket-to-blanket printing machine for conducting offset printing onto both surfaces of a web is taken as one example, but the description will be limited to its blanket section. FIG. 9 is a perspective view illustratively showing a construction of a principal section of a blanket-to-blanket printing machine using a tubular blanket, and FIG. 10 is a cross-sectional view corresponding to FIG. 9.
As shown in FIGS. 9 and 10, a pair of blanket barrels 1 are disposed in parallel to each other to be in an opposed relation to each other in a state where a web 2 is interposed therebetween. Further, a plurality of compressed air discharging openings (not shown) are made in circumferential surfaces in the vicinity of end portions of the blanket barrels 1, and tubular blankets 3 are respectively fitted over the overall circumferences of the blanket barrels 1 to cover the compressed air discharging openings. Each of the tubular blankets 3 serves as an elastic layer constructed by piling up a blanket layer made of a rubber, a fiber, a resin, an adhesive or the like on a sleeve made of an elastic material such as a metal having a high stiffness. The tubular blanket 3 always needs to cover the aforesaid compressed air discharging openings in use, and therefore, to say the least of it, the length of the sleeve of the tubular blanket 3 in its axial directions is required to be substantially equal to the length of the circumferential surface of the blanket barrel 1 in its axial direction.
In addition, the blanket barrels 1 are located so that the web 2 is placed between the tubular blankets 3 to be pressed, and further, the blanket barrels 1 are made to be synchronously rotatable in directions of arrows a, b by a non-shown drive unit, respectively.
Furthermore, at portions on upper and lower sides of the web 2, non-shown printing cylinders are disposed to be synchronously rotatable while coming into contact with the tubular blankets 3 on both sides, respectively, and inks and water for a picture design to be put into print are applied onto the circumferential surfaces of the printing cylinders.
With this constitution, when the web 2 is inserted into the gap between the upper and lower tubular blankets 3 to pass through therebetween in a direction of an arrow c, in a manner that the non-shown printing cylinders, made to be brought into contact with the upper and lower tubular blankets 3, rotates synchronously while coming into contact with the tubular blankets 3, the inks put on the circumferential surfaces of the printing cylinders are transferred onto the circumferential surfaces of the upper and lower tubular blankets 3, respectively.
When the pair of tubular blankets 3, disposed to be in an opposed relation to positions on the upper and lower sides of the web 2, rotates in accordance with the movement of the web 2 while pressing it, the inks transferred onto the tubular blankets 3 are further transferred onto the web 2 to accomplish the offset printing on the web 2.
Incidentally, in detaching the tubular blankets 3 fitted over the blanket barrels 1 or attaching it thereon, the inner diameters of the tubular blankets 3 are enlarged in a manner of discharging compressed air into the gap between the tubular blankets 3 and the blanket barrels 1 from the plurality of non-shown compressed air discharging openings made in the circumferential sections close to the end portions of the blanket barrels 1, thereby facilitating the attaching and detaching operations of the tubular blankets 3.
However, in the case of such tubular blankets, the length of the tubular blanket 3 in its axial directions becomes longer than the width of the web 2, and hence, as shown in FIG. 10, if the web width assumes A or B, the tubular blankets 3 positioned above and below the web 2 results in providing contact areas 4 or 4' where the web 2 does not exist on their circumferential surfaces. That is, in the contact areas 4, 4' where the web 2 is absent on the circumferential surfaces of the tubular blankets 3, the tubular blankets 3 positioned on the upper and lower sides of the web 2 are brought directly into contact with each other without intervene of the web 2 notwithstanding that the web 2 is made to be interposed between the tubular blankets 3. Further, the contact areas 4, 4' where the web 2 is absent on the circumferential surfaces of the tubular blankets 3 enlarge as the web 2 has a smaller width.
In addition, if inks or paper particles coming from the periphery are attached onto the web 2 absent portions on the circumferential surfaces of the tubular blankets 3, since the web 2 does not exist in the web 2 absent areas 4, 4' on the circumferential surfaces of the tubular blankets 3, the inks or paper particles attached thereonto can not shift onto the web 2, with the result that difficulty is encountered to move them in a state of sticking onto the web 2.
For this reason, the inks or paper particles attached in the web 2 absent areas 4, 4' on the circumferential surfaces of the tubular blankets 3 are left to be accumulated therein, and dry and develop heat due to the pressing force working between the tubular blankets 3 positioned above and below the web 2, which can damage the tubular blankets 3.
Moreover, if the web 2 meanders, the web 2 may adhere to the ink accumulated in the web 2 absent areas 4, 4' on the circumferential surfaces of the tubular blankets 3, with the result that there is a possibility that the web 2 is wound around the blanket barrels 1 to be cut off.
Furthermore, in case where the cut-off coming from the winding of the web 2 occurs while the printing machine is in a high-speed operation, until the operation stops after the detection of the web cut-off, the web 2 is wrapped around the blanket barrels 1 in large quantities, and the web 2 is additionally accumulated in the gap between the upper and lower tubular blankets 3 in large quantities, which can make it difficult to put the printing machine into operation because of interfering with the use of the tubular blankets 3.
Besides, the tubular blankets 3 have an excellent high-speed printing performance but are expensive, and are troublesome in replacement, and therefore, in the case of interfering with the use of the tubular blankets 3 as mentioned above, a large loss takes place in economy or in working efficiency.